1. Field of the Invention
The present invention relates to a subscriber circuit and, more particularly, to a subscriber circuit which is connected by a pair of subscriber lines to terminal equipment, such as a telephone.
2. Description of the Prior Art
A telephone switch or central office feeds DC transmission current, monitors the conditions of subscriber line loops, converts two-wire and four-wire signals, and serves other various functions. Implementing subscriber circuits at such a telephone switch with LSI technology has recently become practical. LSI technology has heretofore been proposed in relation to subscriber circuits are disclosed in, for example, "LSI for Telephone Subscriber Line Interface Now in a Practical Stage", Nikkei Electronics, May 10, 1982. As taught in this document, a subscriber circuit in the form of an LSI integrated circuit is achievable by use of current mirror type circuitry. This type of circuitry is simple and has only a small number of elements, so that a small chip can be loaded with a great number of functions.
The subscriber circuit is connected to terminal equipment by a pair of subscriber lines. Usually, several to several hundred pairs of subscriber lines are bundled together and laid as a subscriber cable. Hence, various kinds of electric signals propagate through the subscriber cable, depending on the conditions of the individual terminal equipment connected to the subscriber lines. Assume that a high-voltage signal is fed from a certain subscriber circuit to the associated terminal equipment. Then, the high-voltage signal causes a sharp change in the potential within the cable, and such a change in potential is applied to the other pairs of subscriber lines accommodated in the cable as in-phase induced noise via interline capacitance and inductance. To cope with this problem, it has been customary to provide an LSI subscriber circuit with a noise suppressing circuit which, on detecting in-phase noise, outputs a suppressing current proportional to the amplitude of the detected noise to thereby suppress a change in voltage ascribable to the noise.
The prior art circuit described above has an unsolved problem, as follows. Assume that the in-phase induced noise has a great amplitude and cannot be removed unless the suppressing current exceeds the line DC current. Then, a current mirror circuit, which cannot reverse the direction of the current, is unable to routinely suppress the potential change on the subscriber lines, routinely whereby a differential voltage is developed on the lines. As a result, noise is transmitted to the terminal equipment connected to the subscriber lines and the terminal equipment connected to the telephone switch.